The invention is in the field of electronic reproduction technology and is directed to a method and a circuit arrangement for driving laser diodes arranged in close proximity to one another, for example on a common carrier, in laser recording devices, whereby the term laser recording devices is intended to include laser exposers, laser printers and digital printing machines, among others.
In a laser recording device, a laser beam modulated by a video signal is conducted point-by-point and line-by-line across a recording material that is clamped on a materials"" holder movable relative to the laser beam.
Multi-beam recording elements are employed for increasing the recording speed. A multi-beam recording element comprises a plurality of individually controllable laser diodes that generate a plurality of parallel laser beams for the recording.
Strip-shaped laser diode arrangements, what are referred to as laser diode bars, are frequently employed, these being respectively composed of a plurality of laser diodes arranged in close proximity on a shared substrate carrier that have individually electrically drivable emitters.
Since the light power output by the laser diodes is highly temperature-dependant, the heating of a laser diodexe2x80x94due to the extremely slight spacings of the laser diodes on the substrate carrierxe2x80x94can disturbingly influence the temperatures of neighboring laser diodes and, thus, their light power, an effect that is referred to as thermal crosstalk. Due to the slight spacings of the laser beams from one another, further, neighboring laser beams can mutually influence one another, an effect that is called optical crosstalk. The recording quality of a laser recording device is considerably deteriorated due to such crosstalk.
For crosstalk compensation in laser diode arrangements, it is know to either keep the working temperature of the individual laser diodes constant by heating and cooling or to keep the light powers output by the laser diodes constant by regulating their driver currents.
EP 0 738 071 B discloses a method for the operation of a laser diode arrangement wherein correction signals are formed by weighting selected video signals with weighting coefficients stored in a table memory (look-up table; LUT). For the purpose of a crosstalk compensation, the video signals are corrected by the correction signals and the corrected video signals are then converted into the driver currents for the laser diodes.
The known method does not take into consideration that the thermal crosstalk is dynamic, i.e. that the temperature influencing on neighboring diodes occurs with a time curve. For this reason, the known method is not suited for being utilized for fast modulation of the laser beams in a laser recording device.
It is therefore an object of the present invention to improve a method and a circuit arrangement for driving laser diodes arranged in close proximity in a laser recording device such that thermal and optical crosstalk of the laser diodes are dynamically compensated.
According to the method and apparatus of the invention for driving laser diodes arranged in close proximity to one another in a laser recording device, each laser diode is charged by a driver current that determines a light power output by the laser diode. With the driver currents, controlling video signals modulated with information to be recorded are controlled. A first correction unit is connected between a first laser diode forming a crosstalk source and a second laser diode forming a crosstalk sink. The correction unit is charged with the video signal or the driver current of the first laser diode. The correction unit is connected with the video signal or the driver current of the first laser diode. An output signal of the correction unit is employed as a correction signal for the video signal or for the driver current of the second laser diode. A transfer function of the correction unit is determined such that an optimum compensation of crosstalk is achieved between the laser diodes.
The invention is explained in greater detail below with reference to FIGS. 1 through 4.